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Figure 6.6Track for the period of the Afar 'impact'.

activity, with impact pre-dating the eruptions. (Also, as we shall see below, the Parana event left no evidence of track deviation in Africa, so it is perhaps not surprising that the Afar event also gave rise to relatively little deviation of track and rate of movement.)

Figure 6.7 (a) Track of 'Bombay' over the period 72-60 Ma. It can be inferred from the distances between the points representing specific times that there was an abrupt change in velocity of the plate at about 67 Ma. (b) A detail of the track shown in (a) reveals that the velocity of plate motion doubled in a period which is assumed to be about 5000 years.

Figure 6.7 (a) Track of 'Bombay' over the period 72-60 Ma. It can be inferred from the distances between the points representing specific times that there was an abrupt change in velocity of the plate at about 67 Ma. (b) A detail of the track shown in (a) reveals that the velocity of plate motion doubled in a period which is assumed to be about 5000 years.

Deccan Traps

It has been known for several years that India suddenly doubled its rate of northward movement some time in the period 72-60 Ma. However, as far as we are aware, except for the general hypothesis presented by Seyfert and Sirkin (1979), this change of pace has not been explained.

The track of India over the period 72-60 Ma, which represents the plate motion during that period, is shown in Figure 6.7a. This track is derived by identifying a specific geographical feature on the coast of India, near present-day Bombay. As we have noted, the Atlas system assumes that the present-day geographical shapes of islands and continents are maintained back in time, so that the position of 'Bombay' can be identified for specified ages. In Figure 6.7a, the positions of 'Bombay' from 72 to 60 Ma are given in steps of 1 Ma. Here we are particularly concerned with the sudden change in speed of India, so in Figure 6.7b we show the change of position of 'Bombay' in the period from 67.4 to 67.15 Ma. We plotted the positions of Bombay for the period 67.15 to 67.4 Ma, with the points between 67.30 and 67.20 Ma (i.e. a period of 100, 000 years, in steps of 10,000 a, Figure 6.7b).

Let us assume that the change in the rate of motion could be accomplished in 5000 a. In the period between 67.235 and 67.250 Ma, the slowest possible rise-time is represented in Figure 6.7b by the straight line. However, because of the mass of the Indian lithosphere, the inertia of the system will demand that there is a period of increasing acceleration, starting in 67.235 Ma, followed by a steady-state speed, which then gives way to a deceleration before the plate settles down to a near-constant plate speed following 67. 230 Ma. Thus, the rise-time path is likely to follow the S-shaped dashed line shown by the insert in Figure 6.7b. It may be inferred, therefore, that the maximum acceleration may have had a duration of only a few hundred years. The total rise-time of 5000 a is, as we pointed out, an assumption (the rise-time could, perhaps be as long as 50,000 a). However, whichever figure is assumed, such acceleration is far too fast to be explained in terms of mechanisms normally associated with plate movements. For example, we have

Figure 6.8 (a) Change in track of S America which we take to result from the impact that gave rise to the Parana CPB. (b) Track of E Africa for the same period (see text).

noted that the emplacement of plumes and the attendant hotspot may take at least a million years. Hence, a postulated plume cannot explain the acceleration-time that is to be inferred from Figure 6.7b. We merely note here that change of speed can reasonably be attributed to a major impact. We also note that this track is unusual in that it does not show any obvious deviation following impact. The inferred impact date is 67.4 Ma, while the dates for the erupted Deccan Traps are usually about 65 Ma.

(d) Parana

The track (Figure 6.8a) associated with this continental plateau basalt is the most dramatic we have so far seen; and arguably was the event that initiated the opening of the S Atlantic. Between the years 138-135.1 Ma, it will be seen that S America moved generally northward at a rate of 6-8 km Ma-1. However, in the next 1.1 Ma the S American continent moved, on a track of about 290°, a distance of approximately 150 km. This inferred impact may well have been the most energetic event in the Phanerozoic. Turner et al. (1994), using 40Ar/39Ar geochronology, put the age of the erupted basalts between 137 and 127 Ma, which covers our impact date of 135.1 Ma. However, this movement was restricted to S America. The track of E Africa (Figure 6.8b), which is shown on a much more detailed scale than that of Figure 6.7a, merely shows Africa moving serenely northward, without any detectable deviation of direction or rate of movement. This lack of deviation, we suggest, is an indication of the massive inertia, at this time, of Africa and appended continents and subcontinents, which included Australia, Antarctica, Madagascar and India. This constant northward movement helps one understand why the inferred Afar impact track exhibits such small deviation and change in speed of track.

In passing, it may be noted that the Parana eruptive rocks contain material that originated as molten continental lithosphere. It is difficult to explain how such melt material was generated, if it is assumed that the Parana eruptive rocks were generated by a plume.

Antarctic I

The earlier of the two Antarctic PBs, as Heimann et al. (1994) indicated, comprises the Ferrar Group (consisting of the Kirkpatrick Basalt and Ferrar Dolerite) which crop out along 3000 km of the Transantarctic Mountains, the formation of which are considered to be related to the break-up of Gondwana. These authors point out that, although a wide range of dates (from 90 to 193 Ma) have been cited, more recent work, using 40Ar/39Ar geochronology, shows that the eruptive activity along 1200 km of the Transantarctic Mountains occurred within a short interval of less than 1 Ma, within the time-range of 176.6+/-1.8 Ma.

We constructed the track of an easily identified feature on the Atlas 3.3 version of the coast of Antarctica for the periods 170-186 Ma (Figure 6.9a), 179-182 Ma (Figure 6.9b), with a more detailed track for the period 180-180.5 Ma (Figure 6.9c). It will be seen that there was a sharp change in direction of track, at 180.3 Ma, of 15° and that the rate of plate motion slowed by approximately 25 per cent.

It can be inferred that the extreme upper age cited by Heimann et al. (1994) for the eruptive event is 178. 4 Ma. We would infer from the tracks obtained by using the Atlas 3.3 program that there was a major impact at 180.3 Ma. We suggest that these data are within the error bars inherent in the dating technologies currently used.

Antarctic II

The second Antarctic event, relating to extensive magmatism, was established by Brewer et al., (1992) who also used Ar-Ar geochronology methods. It occurs within the Dronning Maud Land Province and has been shown to have an age of 172.4+/-2.1 Ma. These data were obtained from specimens collected in the transition region between the Dronning Maud Land and Ferrar provinces in the Theron Mountains.

We present another track for the period 175-166 Ma in Figure 6.10, from which we infer that a major impact occurred at 169.45 Ma, which gave rise to a change in track of 22° and an increase in rate of movement of 78 per cent.

In this instance, the youngest possible rock-date is 170.1 Ma, while the impact, as derived from the Atlas program, is cited at 169.45 Ma. We suggest that the 'cart before the horse' relationship can be attributed to small differences inherent in date determinations.

Karoo, S Africa

The igneous activity associated with the Karoo appears to be complex and of long duration, extending as it does from about 193 to 178 Ma. Here we shall only consider the initiation of this event at (about) 193 Ma.

The track shown in Figure 6.11 is related to the Atlas 3.3 representation of the bay containing 'Lourenco Marques' (Maputo). It will be seen that, from 200 to 194.85 Ma the track runs almost exactly northward. At 194.85 Ma, however, it turns through an angle of 100°, so that it continues slightly S of W, but the rate of movement remains virtually unchanged.

As with other events, we would take this track to be evidence of a major impact. The discrepancy of the dates obtained from radiometric dating and from the track of 1.85 Ma indicates that, as one would expect, the impact initiated the change in direction, and that the erupted rock took a significant time to reach the surface.

Figure 6.9 (a), (b) and (c) Progressively more detailed tracks for Antarctica I event.

The date of the impact is also in reasonable agreement with the initiation of the detachment of the Madagascar-India block from Africa.

Figure 6.9 (a), (b) and (c) Progressively more detailed tracks for Antarctica I event.

The date of the impact is also in reasonable agreement with the initiation of the detachment of the Madagascar-India block from Africa.

Siberian flood basalt

Located in N Siberia, this plateau basalt body dated at 248 Ma is of vast extent, but, unfortunately, has been relatively little reported in the English language. The track of Siberia for the period from 260 to 240 Ma is 'anchored' at the head of the Tazovska Guba (TG in Figure 6.12), which runs into the larger Obskya Guba, bounded to the west by the Yamal Peninsula, which in turn faces Novaya Zemlya across the Kara Sea.

It will be seen that from 260 to 250 Ma, the track moves in a generally NE direction. At 250 Ma, however, it suddenly changes track through an angle of 70° and shows a six-fold increase in rate of plate movement. As we have seen, such a track is diagnostic evidence of a major impact. Moreover, one may infer that the energy of the impact event was considerable.

Conclusion: all continental flood basalts were initiated by a major impact.

(The dates of the periods of eruption and dates of track change (and so of inferred impact time) are given in Table 6.2. As noted above, one would expect that a major impact event would occur at a little before the earliest datable erupted rocks which could be sampled. Only two of the eight events listed in Table 6.2 do not show such a relationship and this can probably be attributed to short-comings of earlier dating systems used in the Atlas system.)

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